In a liquid crystal display apparatus, the state of the molecular alignment of the liquid crystal is altered by the action of an electric field and so forth to utilize the accompanying change in optical properties for displaying. In many cases, although the liquid crystal is used by injecting into a gap between two substrates, a liquid crystal alignment film is arranged on the insides of the substrates to align the liquid crystal molecules in a specific direction.
More recently, an optically anisotropic medium obtained by curing a polymerizable liquid crystal material in an aligned state has come to be used as an optical compensation sheet (phase difference plate), which is a kind of optically anisotropic medium, between liquid crystal cells and a polarizing plate, and a liquid crystal alignment film is also used as a material for aligning this polymerizable liquid crystal material.
So far, a rubbing film obtained by rubbing a polyimide or other polymer film with a cloth and so forth in one direction is used as a liquid crystal alignment film. However, in the case of rubbing methods, microflaws in the surface of the polymer film caused by mechanical rubbing cause liquid crystal alignment defects, and due to the non-uniformity of the pushing pressure applied during rubbing, uneven alignment occurs, thereby causing the problem of reduced definition of the liquid crystal device.
In addition, an optical compensation sheet (phase difference plate) is frequently used for the purpose of widening the wavelength band and increasing the accuracy of view angle stability, and in such cases, for example, a laminate of a ¼ wave plate or ½ wave plate, or a laminate of an A-plate and a C-plate is used. However, in the case of manufacturing this laminate, namely in the case of repeating the step of curing a polymerizable liquid crystal layer after fabricating a liquid crystal alignment film layer, if the polymerizable liquid crystal layer by rubbing, the apparatus becomes an extremely large scale and preventing continuous fabrication. Thus, there is a need for a method for obtaining a liquid crystal alignment film that enables all liquid crystal alignment film and liquid crystal layer lamination steps to be carried out continuously.
In order to solve these problems, attention has recently been focused on a technology for fabricating liquid crystal alignment films without the use of rubbing. In particular, a photoalignment method for obtaining alignment of liquid crystal by irradiating a film provided on a substrate with light having some degree of anisotropy is expected to be used practically as a result of having mass productivity and being able to accommodate large substrates.
Examples of compounds capable of serving as such photoalignment films include compounds engaged in photoisomerization reactions in the manner of derivatives of azobenzene, compounds having sites where photodimerization reactions occur such as cinnamate, coumarin or chalcones, and compounds causing anisotropic photodegradation of polyimides and so forth.
An azo compound like that represented by the following structural formula is an example of a photoalignment film material that realigns with anisotropic light currently having the lowest radiation dose (to be referred to as sensitivity) as well as superior crystal alignment ability (see, for example, Patent Document 1). Compounds having this azo structure demonstrate the ability to align liquid crystal at a low radiation dose of, for example, 500 mJ/cm2.
However, since photoalignment films using these azo compounds are low molecular weight compounds, they were susceptible to damage by adhesive members such as sealants used in the liquid crystal cell manufacturing stage. In addition, in the manufacturing of a laminated optically anisotropic medium in which the photoalignment film and a polymerizable liquid crystal layer are repeatedly laminated, although this includes a step in which a polymerizable liquid crystal composition solution is coated onto a photoalignment film or a step in which a photoalignment film composition solution is coated onto a polymerizable liquid crystal layer, the previously fabricated liquid crystal alignment film layer or polymerizable liquid crystal layer is susceptible to damage by a solvent and so forth used in these coating solutions, thereby causing the film separation and preventing the obtaining of uniform optical characteristics. In addition, there was also the problem of deterioration of optical characteristics due to deterioration of the optical anisotropic layer caused by high temperatures generated at the liquid crystal cell manufacturing stage or in subsequent steps after obtaining the optical anisotropic layer by curing the polymerizable liquid crystal material in an aligned state.

Compounds are known in which these azo compounds have been acrylated for the purpose of immobilization (see Patent Document 2). After these compounds have been aligned, the polymerized photoalignment film has superior light resistance. However, sensitivity decreases due to the acrylation, thereby making it difficult to realign at a low radiation dose. In addition, optically anisotropic mediums are known in which a laminated film, comprising a photoalignment polymerizable composition layer using these azo compounds and a polymerizable liquid crystal composition layer, is formed on a substrate, and both layers are polymerized in a state in which the liquid crystal composition having the polymerizing groups is aligned (see Patent Documents 3 and 4). This method is preferable for obtaining an optically anisotropic medium capable of introducing a bonding relationship between both of the photoalignment film layer and liquid crystal polymer layer and having superior adhesiveness and durability. However, since an acrylated azo compound is used, the problem of low sensitivity is unable to be solved. In addition, even an optically anisotropic medium using these azo compounds has the problems causing film separation or unable to uniform optical characteristics during manufacturing of optically anisotropic mediums in which a photoalignment film and polymerizable liquid crystal layer are repeatedly laminated.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H5-232473
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2002-250924
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2005-173547
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2005-173548